1. Field of the Invention
The present invention relates to a peak clipping circuit, and more specifically to such a circuit which is provided in a quadrature modulator of a transmitter which is, for example, provided in a base station of a mobile communications system. Still more specifically, the present invention relates to a method of limiting peak levels of in-phase (I) and quadrature (Q) signals. The present invention is well suited for use in, for example, BPSK (binary phase shift keying), QPSK (quadratype phase shift keying), 16-QAM (quadrature amplitude modulation), etc.
2. Description of the Related Art
As is well known in the art, in a mobile communications system such as CDMA (code division multiple access), a plurality of user's signals are multiplexed and then transmitted simultaneously from a base station at the same frequency. As the number of multiplexed subscriber's signals increases, a peak level of the multiplexed signal increases accordingly. Even in such a case, it is necessary to amplify the multiplexed signal without distortion in order to suppress spreading of signal spectrum. One approach to this problem is to increase a maximum output of an amplifier (e.g., power amplifier) for amplifying the multiplexed signal. However, this approach has encountered the drawbacks that the transmitter itself becomes bulky and the power consumption undesirably increases. In order to avoid such drawbacks, it is conceivable to adaptively control in-phase and quadrature signals in response to a control signal which indicates the number of signals multiplexed (viz., actually transmitted).
Before turning to the present invention, it is deemed advantageous to briefly describe, with reference to FIGS. 1 and 2, a conventional technique to limit the in-phase and quadrature signals at a modulator provided in a transmitter.
FIG. 1 is a signal space diagram showing a QPSK modulation scheme. This diagram is well known in the art and thus a description thereof will be omitted for brevity.
FIG. 2 is a signal space diagram for describing the conventional technique for limiting levels of an in-phase and quadrature signals. Assuming that levels of an incoming in-phase and quadrature signals are respectively denoted by Ix and Qx. The amplitude or distance of the point (Ix, Qx) from an original point, which is represented by (Ix.sup.2 +Qx.sup.2).sup.1/2), is first calculated. Subsequently, the amplitude thus obtained is compared with a clipping level A defined by In and Qn. If the amplitude (Ix.sup.2 +Qx.sup.2).sup.1/2 is larger than the clipping level A, each of the levels of the signals Ix and Qx is multiplied by A/(Ix.sup.2 +Qx.sup.2).sup.1/2. Thus, the level limitation is carried out. However, with this conventional technique, a plurality of multipliers is undesirably needed with the result of complex and bulky hardware.